The present invention relates to television circuitry and more particularly, to a circuit arrangement for generating color level automatic control signal through utilization of VIR (Vertical interval reference) signal contained in vertical retrace interval of television signal being transmitted.
Commonly, in color television broadcasting, it has been a tendency that hue, brightness, etc. of the color video information contained in television signal are liable to be varied or deteriorated during transmission of the television signal from a broadcasting station to television receivers of viewers in general. In order to overcome such inconveniences, there have conventionally been proposed, for example, in U.S. Pat. Nos. 3,780,218 and 3,950,780, circuit arrangements for use in color television receivers for automatic hue control and the like through utilization of the so-called Vertical interval reference (referred to as VIR hereinbelow) system, in which the deterioration as described above is corrected by reference signal (VIR signal) included in the vertical retrace interval of the transmitted television signal, and in the United States, several broadcasting stations have already employed such VIR system for transmission of television signal, with the VIR signal included in the vertical retrace interval of the television signal.
More specifically, in the composite video signal shown in FIG. 1, such VIR signal is included, for example, in the nineteenth line of the vertical retrace interval containing twenty-one lines, while the VIR signal includes reference signals such as chroma reference, luminance reference, black reference signals, etc.
For automatic control of the color level with the use of such VIR signal, conventional practice has been such that a luminance reference level L2 as shown in FIG. 3(a) is subjected to sample holding so as to be the same level as a level L'2 in the chroma reference signal period of the VIR signal in a subsequent field for controlling a color process circuit of the television receiver. The known arrangement as described above, however, has a serious disadvantage in that accurate control of tint or color level can not be effected, since two levels having a time difference of one field must be compared, with contents of one of the levels tending to be varied.